This proposal will investigate central motor control mechanisms that underlie voluntary movement. Central control of motor activity is expressed through activation of the peripheral muscles. While the central nervous system activity underlying voluntary movement cannot be directly measured, peripheral muscle activity can be measured. The Dual Strategy Hypothesis, which models central motor control for single-joint movements, was developed through recordings of surface electromyography. This hypothesis describes two classes of motor tasks which we will test in three experiments. Two distinct but complementary techniques, conventional surface electromyography and motor unit action potentials, will be used to measure muscle activity to elucidate central motor control. Experiment 1 will determine the effect of aging on voluntary motor control by comparing the patterns of surface electromyography in healthy aged subjects and young subjects performing Dual Strategy motor tasks. Experiment 2 will characterize the patterns of motor unit recruitment which underlie the surface electromyographic recordings of the Dual Strategy motor tasks in young subjects. Precision Decomposition EMG will be employed to decompose the constituent motor unit action potentials which underlie the Dual Strategy motor tasks in young subjects. Experiment 3 will define some of the effects of aging and central motor disorders on patterns of surface electromyography and motor unit recruitment. The techniques used in experiments 1 & 2 will be applied to healthy aged subjects and compared with results observed in two central motor disorders: Parkinson's disease and stroke. This proposal will lead to the development of techniques that provide simultaneous kinetic, kinematic, surface electromyography and motor unit recruitment pattern descriptions of motor activity. This will enhance studies which seek to elucidate the physiology of normal and abnormal motor activity. Clinical application of these techniques will broaden the focus of conventional electromyography to include quantitative measures of motor unit recruitment and firing and also allow electromyographic assessment of central motor disorders. These techniques could be used to provide objective measures for evaluating motor dysfunction and monitoring therapeutic interventions. In addition, these techniques may be useful for detecting presymptomatic central motor disorders.